The present invention relates to an expansion card for wireless data transmission as set forth in the preamble of claim 1. The invention also relates to an antenna structure for an expansion card for wireless data transmission as set forth in the preamble of claim 10. Furthermore, the invention relates to a method for fitting an antenna structure in an expansion card for wireless data transmission as set forth in the preamble of claim 14.
According to prior art, various electronic devices, such as portable personal computers (PC) are often equipped with an expansion card interface, to which a standard expansion card can be connected. These expansion cards are intended to form a functional unit with the PC. The expansion card may contain the radio parts of a wireless communication device, including an antenna, wherein the PC can, by means of this card-like wireless communication device, communicate with a communication network, such as a mobile communication network (PLMN, Public Land Mobile Network), which can comply e.g. with the GSM standard (Global System for Mobile Communication). The expansion card can also form a network adaptor for a wireless local area network (WLAN), wherein the card comprises the necessary electrical circuits e.g. for signal processing and a transceiver. With the card, the PC can communicate with other PC devices or devices which form an access point of the local area network. Normally, these devices comprise a corresponding expansion card, used as a network adaptor, to which the wireless data transmission connection is formed. Said antennas are used for the transmission and reception of radio-frequency signals, and the signals are transferred e.g. between the radio part and the antenna of the wireless communication device normally by means of conductors and connectors.
One known expansion card is a PC card complying with the PCMCIA standard (Personal Computer Memory Card International Association). The PC cards are fitted to be placed fully inside the PC, but so-called extended PC cards are longer than conventional PC cards. These extended PC cards are placed partly outside the PC, wherein the thickness and design of the PC cards may vary in this part placed outside the PC. In a known manner, also the separate antenna of the wireless communication device is placed in this part.
Other known expansion cards include for example a so-called CompactFlash (CF) card complying with the CFA standard (Compact Flash Association). Other expansion cards also include a so-called Miniature Card, whose size is only 30 mmxc3x9733 mmxc3x973.5 mm, as well as a so-called SmartMedia card, whose size is only 45 mmxc3x9737 mmxc3x970.76 mm, and a so-called MultiMediaCard (MMC), whose size is only 32 mmxc3x9724 mmxc3x971.4 mm.
One known card-like wireless communication device which comprises a transceiver with an antenna is Nokia Card Phone, i.e. a card phone which can be connected to expansion card interfaces of PC card types 11 and Ill. One WLAN card of prior art is presented in patent publication WO 97/49194, wherein the card part can be equipped with a housing part which contains a radiator functioning as an antenna and formed on a flexible printed circuit board (PCB), comprising a planar conical radiator and a ground potential fitted on the same plane. The radiator and the ground potential are formed by means of conductive copper layers. Another known antenna structure is an air-insulated PIFA antenna (Planar Inverted-F Antenna) which comprises a planar antenna plane having normally the shape of a parallelogram, and a ground plane fitted at a distance from and being normally parallel to the same. The planes used can be metal plates and metal coatings of a circuit board. One PIFA antenna structure is presented in patent publication EP 0 867 967 A2, in which an antenna plane made of a metal plate is supported in a wireless communication device above the ground plane. For the support, a special frame is used in which the antenna plane is fixed. One PIFA antenna structure placed in an expansion card is also known from patent publication EP 0 735 609 A1, in which the antenna plane is formed above a circuit board by coating it with a metal layer, and the ground plane is formed correspondingly on the lower surface of the circuit board. Said circuit board forms a dielectric located in an electric field formed between the planes, and the circuit board is further placed inside an expansion card.
When the card is placed fully inside an expansion card interface, the problem is, however, that the operating range of the antenna is considerably reduced or even totally hindered. This is due to the fact that the metal parts of the device surround the card on all sides, interfering in the reception and transmission of radio-frequency signals. Another problem is that the circuit board used as a dielectric increases antenna losses, and moreover, the distance between the planes can only be varied within the limits allowed by the circuit board. Particularly in expansion cards, the size of the circuit board is limited, wherein the antenna placed on the circuit board takes space from other components, such as integrated circuits (IC).
The electrical operation of the antenna is affected by the dielectric placed between the antenna plane and the ground plane. The electrical properties of this dielectric can be described with the dielectric constant ∈r as well as with the loss tangent value. Losses are increased by all lossy structures which are placed in the electrical field between the antenna plane and the ground plane. Other factors affecting the operation of the PIFA antenna include dimensions of the antenna plane, the location of the feed point of the antenna plane and the earthing point, the disposition of the openings of the antenna plane, and the extent of the ground plane. In particular, when the frequency is increased, the losses of the circuit board material increase.
One known PIFA antenna structure is presented in patent publication EP 0 623 967 A1, wherein above a ground plane placed on the upper surface of a circuit board, and separated by an air gap, there is an antenna plane made of a metal plate which is supported to the circuit board by means of a shorting strip and a feeding strip. Furthermore, there are two antenna plates next to each other in different positions, wherein they form a so-called diversity antenna, wherein reflections and fadings of a radio signal can be taken into account in the reception. The dimension affecting the frequency band of the antenna, i.e. the frequency range within which the antenna is arranged to operate, is the distance between the antenna plane and the ground plane. As antenna structures according to the patent publications EP 0 623 967 A1 and EP 0 735 609 A1 are placed inside the expansion card, the problem is, however, that in the thin card e.g. the thickness of the structures must be limited, wherein also the distance between the antenna plane and the ground plane cannot become very large. The planes can be arranged on different sides of the circuit board, but then the circuit board causes power losses in the same way as the structures of lossy material between the planes.
It is an aim of the present invention to eliminate the above-mentioned drawbacks. The expansion card according to the invention is characterized in what will be presented in the characterizing part of claim 1. The antenna structure is characterized in what will be presented in the characterizing part of claim 10. The method is characterized in what will be presented in the characterizing part of claim 14.
It is an essential principle of the invention to place the antenna structure in a housing part, separately from the circuit board, wherein particularly the distance between the planes can be varied considerably more than in the limited space between the circuit board and the cover structures of the card. Furthermore, the whole inner part of the housing part can be utilized for the antenna structure, if necessary, wherein e.g. a diversity antenna can be located more freely, particularly further away, wherein the operating properties are thus improved. A particular advantage is that, thanks to its fixings, the space between the planes can be kept free from support structures, if necessary, wherein antenna losses can be minimized by arranging therebetween a free and unobstructed air gap with an optimal loss tangent value. A particular advantage is also that the distance between the planes can be maximized to increase the antenna efficiency and antenna gain can be maximized. The distance can thus be greater than the thickness of the circuit board used, or the distance between the circuit board and the cover structure. The invention can thus be used to considerably improve the properties of the antenna. In the invention, the antenna structure extends to different sides of the antenna but is simultaneously placed outside the circuit board.
Furthermore, the invention gives particular advantages, because the antenna structure is now placed separately from the circuit board, wherein the antenna structure can be arranged to be easily exchangeable without exchanging the circuit board. If also the necessary electrical contacts are arranged with spring means, it is particularly simple to exchange or to assemble the card. The whole housing part can be arranged to be exchangeable. A particular advantage is also obtained when the bottom structure of the card consists of a metal plate which can also be arranged as the ground plane. Said metal plate can be common to the card part and the housing part. The cover part of the housing part is preferably manufactured of a plastic material with little losses, wherein it does not interfere in the operation of the antenna plane. Electronic devices comprise processors and electrical circuits, and even weak interference signals radiated by them interfere in the operation of an antenna. By means of a housing part acting as a protrusion, the antenna structure can be arranged outside the interface and further away from the device, wherein the operating range of the antenna and the reliability of the card are improved. Because the antenna part is placed in the housing part, it is also possible to use a thin metal sheet in the manufacture of the card part without interfering the operation of the antenna.
The expansion card used as a wireless communication device according to the invention is particularly suitable for use in electronic devices which are suitable for low power radio frequency (LPRF) networks whose uses are wireless local area networks, such as piconets, wherein the operating range is normally 0.1-10 m, even 100 m if necessary. These wireless networks typically operated in the ISM range at a frequency of 2.4 GHz.